A new PEG-containing liquid crystalline side chain block copolymer HBC-PEG550 was prepared from poly(ethylene glycol)-b-poly(2-(diethylamino) ethyl methacrylate copolymer (BC-PEG550) and 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) by molecular self-assembly processes via hydrogen bond formation between amine group of HBC-PEG550 and hydroxyl group of the LC8. The formation of H bond was confirmed by using FTIR spectroscopy. The liquid crystalline behavior of the HBC-PEG550 was investigated by differential scanning calorimeter (DSC) and polarized optical microscopy. Dielectric properties of HBC-PEG550 and HBC-PEG550 has been studied by impedance spectroscopy. Real and imaginary parts of complex dielectric constant, impedance, and energy loss tangent factor for HBC-PEG550 and HBC-PEG550 have been characterized in the frequency range of 100 Hz–15 MHz.
The application of non-covalent or supramolecular interactions, such as hydrogen bonding, to form liquid crystals is an area of considerable scientific interest mainly due to their interesting electrical and optical properties which makes them good candidates for applications in microelectronic devices ranging from optical data storage and nonlinear optics , , , , , , , , , , , .
Hydrogen bonding enables various mesogenic and nonmesogenic compounds to form complexes with rich liquid crystalline character . Depending upon the length of the chain, type of bonding and the functional groups present in the hydrogen bonded liquid crystals exhibit a rich variety of phase polymorphism and hence the physical properties of the hydrogen bonded liquid crystals are correlated with their molecular structure. Carboxylic and benzoic acid groups are widely used as hydrogen bond donors while pyridine moieties are commonly used as hydrogen bond acceptors. Complexes obtained through intermolecular hydrogen bonding include systems consisting of derivatives of carboxylic (or benzoic) acid/pyridine , , , , , , , , , carboxylic acid/2,6-diaminopyridine , , uracil/2,6 diaminopyridine ,  and carboxylic acid/pyridine N-oxide .
Supramolecular chemistry, specifically hydrogen bonding, has been applied to the field of polymer formation and modification. Polymer liquid crystals are a class of materials which combine the properties of polymers with those of liquid crystals. These hybrid structures possess the same mesophase characteristics as ordinary liquid crystals; in addition they retain many useful and versatile properties of polymers. Moreover, a possibility of chemical variations of the main and side chain compositions of the liquid crystalline polymers (LCP) makes them a promising material for various applications.
We report in this paper the synthesis and characterization of a hydrogen bonded side chain liquid crystalline block copolymer consist of polyethylene glycol block and poly(2-(diethylamino) ethyl methacrylate block and investigate the liquid crystalline features, dielectric properties and conductivity mechanism for its possible technological applications.
All chemicals used were analytical grade: Bromo acetylbromide (Aldrich), poly(ethylene glycol) methyl ether (PEG 550) (Aldrich), 2-(dimethylamino) ethyl methacrylate (Aldrich), triethylamine (Aldrich), CuBr2 (Aldrich), CuBr (Aldrich), N,N,N′,N″,N″-pentamethyl diethylenetriamine (PMDETA) (Aldrich), ethylenediamine tetraacetic acid (EDTA) (Aldrich), dimethyl sulfoxide (DMSO) (Merck), 4′-hydroxy-4-biphenylcarbonitrile (Merck), 8-chloro-1-octanol (Across), anhydrous K2CO3 (Fluka), NaOH pellets (Merck), N,N-dimethylformamide (DMF) (Merck), 1-methyl-2-pyrrolidone (NMP) (Merck), tetrahyrofuran (THF) (Aldrich), and ethanol (Merck) were used as received.
Spectroscopic characterizations of the polymers were carried out by FT-IR (Thermo Scientific Nicolet) and NMR Agilent VNMRS 500 MHz Nuclear Magnetic Resonance.
LC behavior of the polymers has been investigated by polarized optical microscopy (POM) using Leica DM2550P equipped with a LTSE350 Liquid Crystal Prosystem TMS 94 Hot Stage. Samples have been inserted between two thin glass cover slips with a heating-cooling rate of 10 °C/min.
Transition temperature and phase transition enthalpies have been determined by Differantial scanning calorimeter (DSC) using Perkin-Elmer Pyris6 calorimeter at a heating rate of 10 °C per minute under nitrogen atmosphere. Dielectric properties of polymers have been characterized by an impedance analyzer (HP 4194A) in the frequency range of 100 Hz–15 MHz.
Preparation of poly(ethylene glycol) methyl ether based macroinitiator (MPEG-550)
Bromo acetylbromide (10.33 mmol, 0.90 mL) in 10 mL of dry THF was added dropwise to a stirring mixture of triethylamine (13.20 mmol, 2 mL) and poly(ethylene glycol) methyl ether (10 mmol, 5 g) in 40 mL of dry THF at 0 °C. The reaction was continued under stirring for 18 h at room temperature. The solution was filtered, the solvent was evaporated, and the PEG based macroinitiator was precipitated in cold diethyl ether. The macroinitiator was filtered and dried under vacuum.
The synthesis of 2-(diethylamino) ethyl methacrylate block poly(ethylene glycol) methyl ether copolymer by ATRP Method (HBC-PEG550)
CuBr2 (0.0032 g, 0.014 mmol), PMDETA (0.060 mL, 0.287 mmol) were dispersed after ultrasonication in NMP (3.0 mL) for 5 min. 0.2473 g of PEG based initiator was added and the mixture was deoxygenated via nitrogen gas. Then CuBr (0.021 g, 0.15 mmol) and 2-(dimethylamino) ethyl methacrylate (DEAEMA) (4.0 mL, 23.73 mmol) was introduced into the flask under nitrogen flow. Copolymerization reaction was performed at 60 °C for 18 h. After reaction, the mixture was filtered and was interact with EDTA solution to remove copper catalyst. The product was dried under vacuum at room temperature for 24 h and the yield was 5.06 g.
1H-NMR (550 MHz, CDCl3): δ(ppm): 0.82–0.84 (d, 3H, –C–(CH3)), 0.99–1.01 (t, 6H, N(–C–CH3)2), 1.75–1.84 (m, 2H, –(CH2–C–)n), 2.54 (m, 4H, N(–CH2–C)2), 2.67 (t, 2H, C–CH2–N), 3.32 (s, 3H, CH3OCH2CH2O–), 3.58–3.60 (m, 16H, –OCH2–CH2–O–), 3.96 (t, 2H, –CH2CH2OCO–).
Determination of the amine content of the copolymer
A sample of the above product (0.1 g) was mixed with 20 mL of 0.10 M HCl solution and stirred with a stirring bar for 24 h at room temperature. The mixture was filtered and unreacted acid content was determined by titration of 5 mL of the filtrate with 0.1 M NaOH solution in the presence of phenolphthalein color indicator. Total amine content of the polymer was calculated as about 16.09 mmol nitrogen per polymer.
Synthesis of 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8)
Three gram (15 mmol) of 4′-hydroxy-4-biphenylcarbonitrile was in 200 mL of DMSO in the presence of 2 g (14.5 mmol) of K2CO3 anhydrous as acid scavenger. 3.4 mL of (20 mmol) 8-chloro-1-octanol was added drop wise to a stirring mixture under nitrogen at 110 °C. The reaction mixture was heated at 110 °C for 3 h. After this process, the reaction mixture was added drop wise to 400 mL of 10 % NaOH solution at room temperature and filtered. The resultant compound was dried at 40 °C in vacuum. It was re-crystallized from ethanol. Product was obtained in the form of white crystals and dried under vacuum. (Yield: 52 %) IR ν(cm−1): 2224(CN), 3354(OH). 1H-NMR (550 MHz, CDCl3): δ(ppm): 1.2–1.8 (m, 12H, –(CH2)6–), 3.6 (t, 2H, HO–CH2), 3.9–4.0 (t, 2H, CH2–OAr), 6.9–7.0 (m, 2H, aromatic), 7.4–7.5 (m, 2H, aromatic), 7.6–7.7 (m, 4H aromatic).
Preparation of hydrogen-bonded side chain liquid crystalline polymer with LC8
To form H-bond, as a H-bond donor, 0.91 g of 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) (2.81 mmol) and as a H-acceptor polymer, 0.1751 g of block copolymer were dissolved in the DMF in 1 : 1 ratio. Solvent was slowly evaporated at room temperature and resulting solid liquid crystalline polymer was dried under vacuum for 24 h.
Results and discussion
Synthesis and characterization
A new PEG-containing liquid crystalline side chain block copolymer was prepared by molecular self-assembly processes via hydrogen bond formation between 8-(4-cyanobiphenyl-4′-oxy) octan-1-ol (LC8) as a hydrogen bond donor and poly(ethylene glycol)-b-poly (2-(diethylamino) ethyl methacrylate copolymer (BC-PEG550) as a hydrogen bond acceptor.
For this purpose, block copolymer (BC-PEG550) was prepared by ATRP as detailed in the experimental section (Scheme 1).
BC-PEG550 was used to synthesize hydrogen bonded side chain liquid crystalline polymer (HBC-PEG550) as shown in Scheme 2.
The molecular structure of the block copolymer BC-PEG550 and the formation of H-bonds between liquid crystalline mesogen LC8 and BC-PEG550 was confirmed by using 1HNMR and FT-IR spectroscopy, respectively.
In the 1H-NMR spectrum of BC-PEG550 (Fig. 1), the triplet peaks at 3.96 ppm corresponds to the COOCH2 groups, the quartet and triplet peaks at 2.54 ppm, 2.67 ppm and 0.84 ppm corresponds to the N–CH2– and N–C–CH3 groups in the poly 2-(diethylamino) ethyl methacrylate block of the copolymer. The singlet peak at 3.32 ppm and triplet peaks at 3.59–3.60 ppm originate from the CH3O– group and CH2O group in the PEG block of the copolymer.
The IR spectra of the BC-PEG550 and its H-bonded complexes with LC8 (HBC-PEG550) shown in Fig. 2a and b, respectively, are compared in order to analyze the formation of H-bond. The stretching vibrations of the N–C bond of the tertiary amine group in the poly 2-(diethylamino) ethyl methacrylate block appeared 1240 cm−1. Incorporation of LC8 in to the polymer matrix leads to a shift of the band to 1246 cm−1, indicating complementary hydrogen bond formation between tertiary amine group and hydroxyl group.
Thermal and liquid crystalline phase behaviors of LC8 and HBC-PEG550 have been investigated by DSC and POM. The DSC (second heating) thermograms for HBC-PEG550 is given in Fig. 3. HBC-PEG550 exhibits two endothermic peaks at 92.22 °C and 103.34 °C which were assigned as glass transition-nematic and nematic-iso transition, respectively.
The phase behavior temperature and associated enthalpy change (ΔH) of LC8 and HBC-PEG550 are listed in Table 1.
|Code||Phase transitionb (°C) Enthalpy (J/g)c|
|LC8||Cr 76.2 (21.32) Cr 90.7 N 112.6 (0.99) I|
|HBC-PEG550||G 86 (84.17) N 107(2.86) I|
aCr, Crystalline; G, glassy; N, nematic; I, isotropic.
bDetermined by POM.
cDetermined by DSC.
Investigation by POM shows that the HBC-PEG550 exhibits an enansiotropic nematic liquid-crystalline phase. The textures of the mesophase have been identified during heating and cooling from isotropic melt at 107 °C as being a Nematic mesophase at 86 °C which is compatible with the DSC results.
Dielectric spectroscopy is a technique capable of probing the molecular motion and electrical properties of polymeric materials. It involves the measurement of the response of dipoles on polymeric main or side chains to a sinusoidally varying voltage. Dielectric spectroscopy (DS) informs that interactions of electrons, atoms, dipoles and self interfacial depending on frequency in terms of property of polarizable. Dielectric mechanism can be explain some parameters such as, dielectric constant, impedance, and dissipation factor. Alternative current (AC) measurements is one of the most effective techniques for dielectrical characterization of material which is known as dielectric spectroscopy.
The complex dielectric constant ε is expressed in terms of the real ε′ and imaginary ε″ components, which represents the stored and dissipated energy components of the material, respectively. In the complex plane, the complex dielectric constant for liquid crystal can be expressed as ,
Angular frequency ω referred to by the radial frequency, ω=2πf, where f is the frequency (measured in hertz) .
The complex impedance Z(ω) is commonly used to separate the surface and bulk material properties. The complex impedance and its complex conjugate of Z(ω) for express the sample,
respectively. In these equations, ω is the angular frequency, Z(ω) and Z*(ω) are the complex impedance and its complex conjugate, respectively. Z′(ω) and Z″(ω) are the real part, and the imaginary part of the complex impedance, respectively .
The tan δ is given by where δ=90−φ and φ is the phase angle. Dissipation factor (tan δ) is also a typical dielectric parameter.
The real ε′ and imaginary ε″ dielectric constant values decrease with increasing of frequency.
The dielectric constant and dielectric loss variations against frequency for BC-PEG550 and HBC-PEG550 samples are given in a single graph as seen in Figs. 4 and 5, respectively. The ε′ and ε″ is decreasing with the frequency. The ε′ and ε″ have high values due to LC electrode polarization effects. Also dielectric strength Δε has been calculated from Fig. 4. In the dielectric spectroscopy, the dielectric relaxation strength Δε is denoted as;
where, ε∞ and εS are high and low frequency dielectric constants, respectively and their calculated values are given in Table 2.
Here, the dielectric strength (Δε) value of HBC-PEG550 is lower than BC-PEG550. This situation is related to the polarization effect due to the molecular orientation of the liquid crystals.
Frequency dependency of electrical impedance for BC-PEG550 and HBC-PEG550 can be shown from Fig. 6. Frequency evolution of the impedance in log-log graph for all samples have been shown in Fig. 6. The electrical impedance decrease exponentially linearly with increasing of frequency for BC-PEG550. The impedance values are linearly decreasing with frequency for HBC-PEG550.
Frequency dependency of the AC dissipation factor/energy loss for BC-PEG550 and BC-HBC-PEG550 are shown in Fig. 7. According to dissipation factor plots, the BC-PEG550 shows the first relaxation times at low frequencies for and HBC-PEG550 shows the first relexation times at low frequencies. On the other hand, HBC-PEG550 shows the first relaxation times at low frequencies and second relaxation times at high frequencies.
Here we report the formation of a PEG-containing liquid crystalline side chain block copolymer by molecular self-assembly processes via hydrogen bond formation. The characterization of HBC-PEG550 has been confirmed using FTIR, DSC, and POM studies. The HBC-PEG550 shows a transition temperature at 107°C with an enthalpy change of 2.86 J/g corresponding to the transition from liquid crystalline nematic phase to isotropic phase. The dissipation factor peaks increase with adding LC8 (HBC-PEG550) and these peaks which are shielding through the high frequencies.
A collection of invited papers based on presentations at the 16th International Conference on Polymers and Organics Chemistry (POC-16), Hersonissos (near Heraklion), Crete, Greece, 13–16 June 2016.
We are thankful to Istanbul Technical University for financial support under BAP project 39945.
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